Abstract

A ubiquitous feature of neural responses is their dependence on stimulus context. One prominent contextual effect is the reduction in neural response size with stimulus repetition, known as “adaptation”. As adaptation is often stimulus-specific, it has been used in visual neuroimaging studies to probe mechanisms of stimulus representation that would otherwise be hidden due to the limited spatial resolution of the available measurement techniques. However, work on the visual system has suggested that stimulus-specific adaptation may not only reflect stimulus representations, but may itself also modify representational information. The four studies described in this report examined the effects of stimulus context on auditory cortical responses using electroencephalography (EEG).

The first study used adaptation to examine the neural representation of musical pitch in auditory cortex. Whilst pitch is often treated as a single dimension, namely, the repetition rate of the stimulus waveform, in music, pitch actually has two dimensions: pitch height (the octave in which a note resides) and pitch chroma (the position of the note within an octave). The current study provided evidence for an explicit representation of pitch chroma in an anterolateral region of non-primary auditory cortex.

The second, third and fourth studies examined the auditory “mismatch response” (MMR). The MMR refers to the increase in response size to a stimulus when it is presented infrequently (as a “deviant”) compared to when it is presented frequently (as a “standard”). The second study found that the MMR could not be fully accounted for by a passive release from adaptation. Instead, the MMR seemed to reflect a sharpening of the neural representation of the adaptor stimulus with repeated presentation. This suggests that the MMR may be involved in perceptual learning.

The third study examined the time courses of the contextual effects on neural responses. Both short- and longer-term effects were observed, with the effects differing between the different components of the auditory evoked response. Notably, the N1 component was influenced by complex effects that seemed to partially reflect the longer-term probabilities of certain short segments of the stimulus sequence, whereas the P2 was influenced by a strong suppressive effect with a remarkably short time course. The fourth study examined whether the contextual effects on auditory-evoked transient and sustained responses are sensitive to the absolute, or the relative, stimulus probabilities. For the transient N1 response, the most striking finding was that adaptation was broadly tuned for deviant stimuli, but sharply tuned for stimuli that were, in terms of their relative probabilities, standards. In contrast, the sustained response appeared to be influenced by a different effect, which facilitated responses to deviant stimuli.

The current results suggest that contextual effects differ vastly between different deflections of the auditory-evoked responses, that they include effects that are both complex and long-lasting (of the order of ten seconds or longer), and that they involve not only suppressive, but also facilitatory effects.